1. Field of Invention
The present invention relates generally to magnetic data storage devices, and particularly to read/write heads for use in such devices. The invention particularly teaches new method and apparatus for a new thin film head for high density magnetic disk drives for use in computer data storage.
2. Description of the Prior Art
A read/write head is used to record and store information on a rotating magnetic disk or magnetic tape, and also to read back the stored information. A typical mass storage devices store information on spinning magnetic disks, the information being recorded in the form of transitions in magnetic flux on the magnetic surface of the disk. In particular, the data is recorded in a plurality of tracks, with each track being a selected radial distance from the center of the disk. The number of transitions per inch along the track defines the bits per inch (BPI), and the number of tracks per inch along the radial distance defines the tracks per inch (TPI). The product of the BPI and the TPI defines the areal density stored in the magnetic film on the disk. A read/write head flies in close proximity to the disk surface and is held in approximate radial position over the disk by an arm. Under the control of the system's processor unit the arm can move the read/write head to the appropriate track in which the data is recorded so that it may be read, or into which data is to be written.
A commonly used inductive read/write head comprises two pole pieces formed from a magnetic material and a write coil. At one end, the pole pieces are touching and at the other end there is a slight gap between the pole pieces. The head is positioned so that the gap is directed towards the disk surface. When electric current is impressed on the coil, a magnetic flux is generated, which is impressed upon the pole pieces. The width of the poles along the track direction corresponds to the width of the track in which information is recorded. The smaller the pole width, the narrower is the track, thereby increasing TPI, which is the trend of the future in high density recording.
At the gap, the magnetic flux is directed through the magnetic material in the adjacent disk surface to thereby impress magnetic flux therein. A thinner gap head writes narrower transitions, thereby increasing BPI. Higher BPI and higher TPI increases areal density, thereby increasing information storage in the disk surface. This higher storage density is very desirable for the miniaturized disk drives with high information storage capacity, needed for the popular computers, e.g., PCs, workstations, laptops, notebooks, and the like.
When data is being written onto a disk, the write coil is energized with a varying current pattern which corresponds to the data to be written. The varying current results in the generation of the corresponding pattern in the magnetic flux which the head applies to the surface of the rotating disk. Since the disk moves relative to the head, the magnetic flux on the disk surface also varies along the length of the arc traversed by the head on the disk.
When the data is read, the head flies over the arc of the disk surface in which the data was written. A small amount of flux from the disk permeates mostly into the poles of the head. The flux in the head varies in response to the pattern of flux recorded on the disk. The varying flux results in the generation of varying voltage in the coil, which, in turn, is sensed as the previously-recorded data.
In a known process for making thin film heads, an insulating base layer of, for example, A1203, is deposited on a substrate. Because this base layer is insulating, a sputtered seed layer of a material such as NiFe is applied to the base layer. Photoresist is then spun over the seed layer and a pole piece pattern is then formed by photolithographic techniques. U.S. Pat. No. 4,900,650 discloses a pole piece pattern in which the narrower poletip (defining the trackwidth) is connected to a bigger pole structure (the "bulb"), and NiFe film is deposited everywhere, except a 10 to 15 micron band surrounding the pole piece geometry, by through-mask electroplating. This approximates to a sheet plated NiFe film which has better uniformity and control of NiFe composition and thickness as compared to polepiece geometry without the bulb. Because the neck region in the pole tip is narrow, this pole piece has properties inferior to those of a pole piece formed from a sheet plated NiFe film. For higher TPI heads the poletip width has to be narrower than 6 microns. As the poletip becomes narrower, the effect of this inferiority becomes greater.
The existing art of fabricating magnetic pole pieces of a thin film head includes a through-mask plating process. U.S. Pat. No. 4,695,351 discloses that a through-mask plated pole with a poletip in the range of 5 to 10 microns requires an orienting field of at least 5000 Gauss. In order to define the same easy-axis everywhere in the magnetic film, the magnetic field is required to be only high in magnitude but also uniform in direction, particularly in the volume where potentially functional devices have to be plated. The need of high magnitude (5000 Gauss) and uniformity in direction of magnetic field requires a large magnet in the plating cell. This makes the plating cell very bulky, inconvenient and expensive. Even with this, not too large an area can be plated with uniformly defined easy-axis film, because the complexity and expense increases multifold with the increase in the volume, where the field has to be large and uniform in direction. This limits the size of the wafer and the number of the wafers to be plated at a time, correspondingly restricting the manufacturing throughput. Furthermore, heads produced by through-mask plating have variations in readback signal amplitude and in the readback pulse shape due to undesirable magnetic domains in the formed head.
Besides the magnetics problem discussed above, there is another problem in fabrication of a narrow track thin film head. A typical thin film head is fabricated by through-mask plating of the first magnetic pole (P1) on a previously sputtered seed layer for plating. After plating, the seed layer is removed by sputter etching from the moat area surrounding the pole geometry, the pole geometry is protected by photoresist via a photolithographic process and current thieve areas are chemically etched away, and the photoresist protection is removed by dissolving the photoresist in a solvent. This defines the NiFe magnetic pole piece P1, while the second magnetic pole piece (P2) is formed later. Then a 0.3 to 0.6 micron thick alumina layer is formed on P1 to define the read/write gap. Alumina in the back closure area (away from the poletip area) is then etched away via photolithography and chemical etching processes, to enable contact between the P1 and P2 pole when the P2 pole is formed. Copper coils (C), separated by a hard-baked photoresist polymer layer, are then deposited along with the bonding pads. The coils and the adjacent layer of the P2 pole are separated by the hard-baked photoresist layer. This builds a high topography over which P2 must be aligned over the bottom pole (P1). The second magnetic pole (P2) is then deposited on the then built structure via a through-mask plating process in the same manner as the pole P1 was defined. Ideally, the poletip of P1 and the poletip of P2 are of the same dimension and are accurately aligned one over the other, but in practice, this is not possible due to photolithographic limitations and the high topography. Therefore, in a practical thin film head, the pole width for P1 in the poletip region is made wider than the poletip width of P2. In thin film heads, the P1 poletip is made about 1.5 to 2 microns wider than the P2 poletip so that the P2 poletip can be accurately defined within the P1 poletip width. The trackwidth of such heads is not defined by the P2 poletip width but is a compromise between the P1 poletip width and the P2 poletip width.
As track width becomes narrower, as is the case in the present and the future trend of the high density magnetic recording, the offset between the P1 and P2 poletip widths, e.g., about 2 microns, makes a severe compromise in the performance of the head. One approach of making such a narrow track head is first to complete the processing steps up to forming of P2 pole. The P1 poletip width can be much larger than the P2 poletip width. In this approach, the trackwidth is defined by photo patterning the tip region and ion-beam etching the unprotected poletip region. This has been discussed by T. Nakanishi et al in the IEEE Trans. Magn., MAG-15, pp-1060. Even in this approach, however, the consistent definition and alignment of the narrow track poletip is a problem in view of the limitations in exposing the thick photoresist formed in the poletip region arising from the high topography of the head.
It is an object of the present invention to provide a new and improved technique for fabricating a thin film head for high track density recording.
It is another object of the present invention to provide a technique for manufacture of a thin film head defining an easy axis parallel to the track in the narrow poletip without a need of a very high magnetic field in the plating cell.
It is another object of the present invention to provide a technique for defining narrow poletips of equal widths of P1 and P2.